Project Summary Lack of consensus regarding the optimal treatment approach for men with low-risk prostate cancer has resulted in significant variation in practice patterns (Cooperberg et al. 2010). In recent years, a high and increasing proportion of men with low-risk prostate cancer have received aggressive treatment, including intensity modulated radiation therapy (IMRT) and prostatectomy performed using the daVinci robot (Jacobs et al. 2013). This trend is concerning because IMRT and robotic surgery are both substantially more expensive than the older technologies they have replaced?start-up costs for each of these advanced technologies hovers around $2 million (Barbash and Glied 2010; Carreyrou and Tamman 2010)?but the new technologies offer no additional benefit in terms of survival for low-risk disease (NCCN 2011, Nguyen et al. 2011). Moreover, the greater use of aggressive treatment for men with low-risk prostate cancer has occurred despite evidence suggesting that more conservative monitoring approaches may be more appropriate for patients with indolent disease. Recent findings from randomized trials of aggressive (radical prostatectomy) compared to conservative (watchful waiting) modalities do not contradict recommendations for conservative treatment of low risk disease (e.g., Bill-Axelson et al 2011; Wilt et al 2012). Several observational studies find low mortality rates among men with well-differentiated disease treated conservatively (Albertson et al. 2005; Klotz et al 2010; Lu-Yao et al 2009). However, these observational studies do not control for patient selection into conservative treatment, despite evidence that patient selection into treatment is significant for clinically localized cancer (Harlan et al 2001). Moreover, previous observational studies have not controlled for physician characteristics, including compensation structure that may affect treatment modality. Previous research has also not fully assessed the consequences of aggressive versus conservative treatment of low- risk prostate cancer for outcomes beyond mortality, including side effects and episode treatment costs. Our proposed research addresses these significant gaps in knowledge using a sample of men age 65 and older with newly diagnosed low-risk prostate cancer during the time period 2005-2014. Our data includes Medicare claims records merged with state cancer registry data from California, Florida, New Jersey, and Texas. Our aims are as follows: Aim 1: To evaluate whether and how provider attributes affect type of treatment received after controlling for clinical factors and patient characteristics. Aim 2: To evaluate whether receipt of aggressive treatment compared to conservative management affects the probability of experiencing adverse health outcomes including death and complications from treatment. Aim 3: To evaluate whether receipt of aggressive treatment compared to conservative management affects cancer-related episode expenditures.